The long term goal of this project is the investigation of mechanisms of photosensitivity in patients who are abnormally sensitive to sunlight. A major aspect of the investigation is based on determining the action spectra; that is, identification of ultraviolet, visible or infrared radiation and the minimal dose of the radiation that will provoke changes in the skin. This is determined by irradiating the skin with narrow wave bands of measured intensity, obtained from high intensity xenon or xenon mercury lamps. Monochromatic light has been obtained with interference filters, but more recently a high intensity tandem prism-defraction grating monochromator has been constructed which has been available for testing since January, 1969. A knowledge of the action spectra of a disease may lead to an understanding of its pathogenesis or even biochemical identification of the photosensitizing substance. This hypothesis is based on the principle that radiant energy is only active after absorption, so that if the active wave length is known, a search can be made amongst substances exogenous or endogenous which are known to absorb them. Photosensitive patients and biological systems are also irradiated with a solar simulator attachment to the xenon arc. This apparatus includes an optical integrator to uniformly diffuse the arc and suitable filters and optics to collimate the beam to simulate a solar spectrum of 290-750 nm. The biochemical and biological research into the cause of photosensitivity includes derangement of porphyrin synthesis in human erythropoietic and hepatic porphyria, including more recently a site visit to Turkey for a 20-year follow up on Turkish porphyria and corresponding experimental porphyrias. More recently the investigator has reevaluated the action spectra of topical psoralens (P) which showed maximum sensitivity at 330-335 nm. and proposes to determine whether this portion of the spectrum would be more therapeutic (and correlate this action spectra on the inhibition of DNA synthesis) in the treatment of psoriasis with UVA (PUVA).